Typically representative of engineering plastics, PAS has superior properties, including heat resistance, chemical resistance, flame resistance, and electrical insulation, and thus the demand therefor is increasing for use in high-temperature and corrosive environments and in the field of electronic products. This polymer is mainly used for computer parts, automobile parts, coatings for parts in contact with corrosive chemicals, and industrial fibers having chemical resistance.
In particular, PAS, which is presently commercially available, is solely exemplified by polyphenylene sulfide (hereinafter, referred to as ‘PPS’). PPS is commercially produced by reacting p-dichlorobenzene (hereinafter, referred to as ‘pDCB’) with sodium sulfide in a polar organic solvent, such as N-methylpyrrolidone. This method is known as a Macallum process, and is based on U.S. Pat. Nos. 2,513,188 and 2,583,941. Although some other types of usable polar solvent have been proposed, only N-methylpyrrolidone is presently useful. This process is conducted using only the dichloro aromatic compound, and sodium chloride (NaCl) is produced as a byproduct.
PPS, obtained by the Macallum process, has a molecular weight of about 10,000˜40,000 and a melt viscosity not higher than 3000 poise. In order to obtain a higher melt viscosity, PPS is subjected to heating to a temperature below the melting point thereof, and post-treatment (curing), in which it is brought into contact with oxygen. As such, through oxidation, crosslinking, and polymer chain extension, the melt viscosity is increased to the level required for general use.
However, PPS, obtained by such a conventional process, has the following defects.
First, because sodium sulfide is used as a sulfur source required for polymerization, a metal salt, such as sodium chloride, as a by-product, is in the polymer by large amount. Even after the resultant polymer is washed, the metal salt may remain at the level of thousands of ppm, undesirably causing the corrosion of processing equipment and problems related to a spinning process upon the manufacture of fibers, as well as an increase in the electrical conductivity of the polymer. Further, from the manufacturer's point of view, the case where sodium sulfide is used is problematic in that sodium chloride is produced as a byproduct in an amount of 52% based on the weight of the material added, and should be discarded in that state even if it is recovered, because there are no economic benefits thereof.
Second, in the post treatment, the properties of a polymer grow worse. That is, the polymer is heavily colored by oxidation and crosslinking, and becomes highly brittle, in terms of mechanical properties.
Finally, as in polymers obtained by solution polymerization, the final form of PPS is very fine power form, which decreases the apparent density thereof somewhat, making it difficult to transport such polymers, and which also causes a lot of inconvenience in the course of the processing thereof.
New processes other than the Macallum process have been disclosed in U.S. Pat. Nos. 4,746,758 and 4,786,713 and related patents. These patents refer to the production of PAS from a diiodo compound and solid sulfur, instead of the dichloro compound and metal sulfide in the conventional process, through direct heating without the use of a polar solvent. This method includes two steps of iodization and polymerization, in which the iodization allows an aryl compound and iodine to react to thus obtain a diiodo compound, and the polymerization allows the diiodo compound thus obtained to react with solid sulfur, thereby producing PAS having a high molecular weight. As such, iodine, which is in a vapor form during the reaction, is recovered, to allow it to react again with an aryl compound, and hence, iodine acts as a catalyst in practice.
This method may solve the problems of conventional processes. Because iodine is a by-product, it does not increase electrical conductivity, like the metal salt, and facilitates recovery from the reactant, thus making it easy to decrease the content thereof in a final product so that it is lower than the content of the metal salt according to the conventional process.
Also, the recovered iodine may be reused in the course of iodization, and there is almost no waste. Second, because no solvent is used in the course of polymerization, pellet products may be produced, like conventional polyester products, thereby avoiding problems related to the use of fine powder products. Finally, this method may father increase the molecular weight of final PAS, compared to the conventional process, and therefore, there is no need for post treatment that undesirably worsens properties.
However, the novel process above is notably disadvantageous because it entails the following two problems. First, in the case where iodine remains in a molecular form, it may harm the processing equipment when contained even in a small amount in a final PAS product, due to its corrosiveness. Second, because solid sulfur is used in the course of polymerization, a disulfide link is present in final PAS, undesirably deteriorating thermal properties, including the melting point.